Following the mass extinction that originally defined the top of T.f the Letter Stages were defined on percentages of extant moluscs rather than larger forams. Thus the T.g and T.h have fallen into disuse as mollusc-based biostratigraphy has diminished in practical application. To the original workers this mixture of fossil groups was a practial transition from mollusc stratigraphy, which had been used to define the Eocene, Miocene, Pliocene and Pleistocene, to the more abundant and easier to work with larger foraminifera. This transition also emphasises that the Letter Stages are best regarded not just as assemblage biozones but as Indo-Pacific equivalents to the European biostratigraphic epochs of Eocene through Pleistocene.
It can now be recognised that after the mass extinction at the original top T.f - now called the lower T.f to Upper T.f boundary following Adams (1970) - the Upper T.f (ex "T.g") was a period of relative stasis in evolutionary terms for the diminshed larger foraminifera. It was only in latest Miocene times that a slow radiation of new larger foram types appeared. This was a much slower event than the rapid raditation after other mass extinctions, such as after the end of T.a. In the warmer times at the beginning of T.b many new body forms appeared, and were often widespread and common, in just one or two million years. From the end of the Miocene a trickle of new morphotypes appeared as uncommon members of the carbonate assemblages over three or four million years (Calcarina, Alanlordia, Quasirotalia, Bacculogypsina, Schlubergerella etc.
The images shown here are reduced to about 1/5 their original resolution and highly compressed as JPG files. Original images available on request, as TIF files from 120 to 400 MB in size
